Without limiting the scope of the invention, its background is described in connection with Frequency Selective Surfaces (FSS). The term, periodic surfaces, is used to describe a broad range of objects which include everything from periodic arrays of antennas and apertures to optical diffraction gratings, at wavelengths from microwave to X-ray. One type of surface is the wire grid, useful for filters and polarizers. Periodic surface filters, also known as Frequency Selective Surface (FSS) are a further development of wire grid technology. They are composed of appropriately shaped conductors in thin layers on a dielectric substrate. The spacing between elements in the layers can be less than a wavelength. In the optical wavebands, this results in transmission of a single diffraction order, instead of the multiple orders transmitted by larger early technology.
IR filters can be made using conducting surfaces composed of Frequency Selective Surfaces (FSS). Most prior developments are for fixed frequency structures (i.e. the resonance or resonances, giving either bandpass or bandreject frequency characteristics, cannot be time varied or modulated or tuned).